If people running the nuclear power plants who are Ukrainians are shot by the Russians

 Or they run away in order not to die by the Russians this leaves nuclear power plants with no one who knows how to keep them safe at all. IN the fog of war a meltdown would be pretty easy to imagine at one of the 16 power plants if you include Chernobyl which they covered with cement in 1986 but is still just as dangerous if it isn't constantly watched and monitored. So, any of the other 15 Soviet style nuclear power plants could also become other Chernobyls. And since there might be dead scientists and engineers who fought the Russians at these power plants it is easy to imagine NO one knowing what to do or that there is even a problem in the area of that particular one of 16 potential problems during a war.